In an electronic article surveillance (EAS) system of known type, a transmitter-receiver arrangement is disposed aside an area to be controlled and transmits a first high-frequency signal into the area. A separate transmitter furnishes a second signal of substantially lower frequency (commonly referred to as the E-field or electrostatic field signal). Reradiators, typically comprising a dipole and a nonlinear element, are responsive to the incidence thereon of both transmitted signals to transmit a composite thereof and detection of such composite signal in receiving apparatus indicates the presence of the reradiator (security tag) in the controlled area. Such system is further described in U.S. Pat. Nos. 3,895,368 and 4,139,844, commonly-assigned herewith and incorporated herein by this reference.
Conductive objects create a problem for security tags which make use of electrostatic energy for detection. Because electrostatic fields present extremely high driving-point impedances to circuit elements, any conductive object creates a short circuit path. Consequently, circuit elements are at equal electrostatic potential, and no resulting voltage can exist between any two points in such an environment.
Virtually all electronic circuits in common practice are comprised of components that are essentially low-impedance or near short circuits to electrostatic fields. In the typical security tag above described, voltage differential across the nonlinear circuit element, which may be a diode, is essential to generation of the composite signal to be reradiated. Evidently, signal generation would be thwarted by the equal electrostatic potential which would be imposed thereon by a conductive object in close proximity to the tag.
In order to minimize the short-circuit effects of conductive objects, electrostatically driven high-frequency tags heretofore have been required to be placed remotely from the objects, which greatly increases the thickness requirement of such a combination of tag and object.